This invention relates to a method for the pneumatic transport of a chargeable resin powder such as vinyl chloride resin powder.
Pneumatic transport is often employed for conveying a large quantity of resin powder such as thermoplastic resin powder. The pneumatic transport is generally carried out by taking air from the ambient atmosphere, compressing the air by a boot blower, and optionally cooling the air.
One class of thermoplastic resin powder includes chargeable resin powders such as vinyl chloride resins, ABS resins and SBC resins. The pneumatic transport is also employed for conveying a large quantity of such chargeable resin powders.
However, during the pneumatic transport of chargeable resin powder, the resin powder tends to build up electrostatic charges due to friction or the like. Electrostatic charging gives rise to the following problems. (i) The resin powder itself is so reduced in bulk density that the transport becomes efficient. (ii) The resin powder forms aggregates or bridges within the transport pipe, reducing the transport efficiency and still worse, causing the pipe to be clogged to substantially restrain pneumatic transport. (iii) After the resin powder is transported to the tank, the powder forms aggregates or bridges in the tank. Since the resin powder is not permitted to fall under gravity upon withdrawal of the resin powder from the tank, it is necessary to apply vibration to the tank in order to withdraw the resin powder from the tank.
An object of the invention is to provide a novel and improved method for the pneumatic transport of chargeable resin powder which prevents the chargeable resin powder from building up electrostatic charges during the transportation and thus has solved the above problems (i) to (iii).
We learned that when a chargeable resin powder is transported by means of a conveying airstream obtained by compressing air in a boot blower and cooling to about 10 to 70xc2x0 C. in an after cooler, moisture in the resin powder volatilizes during transportation so that the chargeable resin powder becomes more and more readily chargeable. This, combined with friction among resin particles, causes the chargeable resin powder to readily build up more electrostatic charges. We have found that by using an air/steam mixture obtained by admixing the conveying air with steam, and properly adjusting the humidity of the air/steam mixture, the electrostatic charging of the chargeable resin powder is minimized, thereby preventing a lowering of bulk density, formation of aggregates or bridges, and clogging of the pipe due to electrostatic charging of the chargeable resin powder.
The invention provides a method for transporting a chargeable resin powder by means of an airstream. An air/steam mixture obtained by admixing air with steam is used as the airstream. The chargeable resin is typically a vinyl chloride resin.
In a preferred embodiment, the air/steam mixture is adjusted so as to meet the following relationship:
0xc2x0 C. less than (T1xe2x88x92K1)xc2x0 C.xe2x89xa660xc2x0 C. 
wherein T1 is the temperature of the mixture which has been used in pneumatic transport of the powder and separated from the powder and K1 is the dew point of the mixture.